I've used scopes for years on jet engine inspections. They're harder to use than you expect. Because you're close up to the surfaces, and looking at a very small area, it's quite difficult to figure out where you are, and to judge what you're looking at. Ok, there's a scratch. How deep is that scratch, how long is it? Even with a ruler built into the lens, it's darn hard to tell.
It's sorta like trying to examine something with one of those extension mirrors. Hard to do, and even harder to judge the condition of things you're looking at.
A cheap scope will look straight ahead, out the end of the fiber optic strand. That lets you look at the face of the piston, which isn't very usefull.
A step up will have a mirror you can clip onto the tip. This lets you look sideways. Now you can look at the piston wall.
To look backwards at the cylinder top and valves, you'd need to make your own mirror, as I've never seen one that did this.
Then there's the matter of light. Really cheap ones may not have a light, so you can't see anything. Most will have the light running through the optic cable strand, so the light points straight ahead. This means when using a mirror you're shining the light on the mirror you're looking at. Tends to defeat your ability to see in the mirror.
Really nice scopes have control cables along the fiber optic cable to bend it around. This will look backwards, *if* it can turn tightly enough in the cylinder bore. That's a small hole, so many will not be able to do it.
Used scopes often have broken strands. Each broken strand is a black pixel in the display. The more broken strands, the more blacked out pixels you've got. Beware of this with e-bay used scopes.